GMR sensor with a varying number of GMR layers

ABSTRACT

A position sensor for non-contact position measurement includes a sensor magnet and a sensor body. The sensor body is formed from a magneto-resistive material and is given a two or three dimensional geometrical shape to achieve a desired sensitivity function. The desired sensitivity function results from a variation in one or more of the sensor body dimensions. By forming the sensor body of magneto-resistive material into different geometrical shapes like a simple wedge, a double wedge, a circular tapered form or an arbitrary shape, a desired sensitivity function is obtained.

TECHNICAL FIELD

The present invention relates to tailoring the shape of amagneto-resistive material, and more particularly to a design of theshape of the magneto-resistive material to obtain a new type of positionsensitive sensor.

BACKGROUND

The position of a moving object is often determined by means of thereadout from a resistive sensor, usually of potentiometer type, which ismechanically connected to the object to be monitored.

In order to reduce the wear and thereby crease e reliability, it isdesirable to eliminate the sliding friction encountered in the standardresistive sensors. Non-contact methods using e.g. inductively coupledcoils is currently being introduced as replacement for the potentiometersensors. However, these are more complex and therefore more expensive.

In recent years novel types of magneto-resistive materials with muchhigher sensitivity to moderate changes in magnetic fields have beenfound. These new materials showing giant magneto-resistance (GMR) orcolossal magneto-resistance (CMR) make possible new types of positionsensors.

In a document U.S. Pat. No. 5,475,304 is disclosed a giantmagneto-resistant sensor including at least one layered structure. Thelayered structure includes a ferromagnetic layer having a fixed magneticstate, a second, softer magnetic layer, and a metal layer interposedbetween and contacting these two layers. The sensor also includes one ormore indexing magnets for inducing a domain wall, at a measuredposition, between regions of nonaligned magnetic fields in the softermagnetic layer. By measuring the resistance across the magneto-resistantsensor a displacement of one workpiece, carrying the sensor, will bemeasured relative to another workpiece carrying an inducing means.

Yet another document U.S. Pat. No. 5,627,466 discloses a positionmeasuring device having a sensor, the output signal of which is afunction of the distance between a graduation and a scanning unit.Magneto-resistive elements, which scan the graduation, are disposed inthe active branch of a potentiometer circuit. The voltage over theactive branch is taken as the distance-dependent signal and is used tocontrol the amplitude of the position-dependent scanning signalsgenerated by scanning the graduation.

However, there is still a demand for non-contact sensor devices forposition measurement offering a sensitivity function adapted to theparticular application.

SUMMARY

The object of present invention is to disclose a device, which forms aposition sensor for non-contact position measurement The devicecomprises a sensor magnet and a sensor body made of a magneto-resistivematerial, whereby the magneto-resistive material is formed into a bodyhaving two or three dimensional geometrical shape to achieve a desiredsensitivity function. The desired sensitivity function then results froma variation in one or more of the sensor body dimensions.

According to the object of the present invention the sensor bodypresents in different embodiments shapes like a simple wedge, a doublewedge, a circular tapered form or an arbitrary shape to obtain thedesired sensitivity function.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by making reference to the following descriptiontaken together with the accompanying drawings, in which:

FIG. 1 illustrates a basic circuit diagram for measuring the change ofthe resistance in a position sensitive sensor made from amagneto-resistive material;

FIG. 2 illustrates an embodiment with a basic wedge shape for a positionsensitive sensor made from a magneto-resistive material;

FIG. 3 illustrates an embodiment having a double wedge shape for aposition sensitive sensor made from a magneto-resistive material;

FIG. 4 illustrates an embodiment having a circular shape for a positionsensitive sensor made from a magneto-resistive material; and

FIG. 5 illustrates an embodiment having an arbitrary shape for aposition sensitive sensor made from a magneto-resistive material

DETAILED DESCRIPTION

Theory

The electric resistance of a segment of length Δl and an average area Amade up of a material with resistivity ρ is given by${\Delta \quad R} = {\rho \quad \frac{\Delta \quad l}{A}}$

If a magnetic field is applied over this segment the resistance changesby the amount$\quad {{{\delta\Delta}\quad R} = {{\delta\rho}\quad \frac{\Delta \quad l}{A}}}$

the magnitude of this change depends on the material in question and isfor GMR materials typically some tens of percent while for CMR muchhigher values can be obtained in limited temperature ranges. For aconductor of constant cross-section the change is independent ofposition but if A is a function of position, unique position informationcan be obtained. The above described sensitivity to position is utilizedfor instance in a Wheatstone type bridge circuitry consisting of twoidentical standard resistors R and one (or two) magneto-resistiveelements R₁ and R_(x)=R1−δΔR. If one of the magneto-resistive elementsis exposed to a magnetic field over the distance Δl at position x, theresistance of this element changes by${{\delta\Delta}\quad R} = {{\delta\rho}\quad \frac{\Delta \quad l}{A(x)}}$

as a consequence, the voltage between the connecting points A and B(FIG. 1), V_(AB), changes from an initial value of zero to$V_{AB} = {{- \frac{E}{4}}\frac{\delta \quad \Delta \quad R}{R - {{\delta\Delta}\quad R}}}$

For small δΔR this changes to$V_{AB} = {{- \frac{E}{4}}\frac{\delta \quad \Delta \quad R}{R}}$

and the voltage difference is then directly related to the position.$V_{AB} = {{- \frac{E}{4R}}{\delta\rho}\quad \frac{\Delta \quad l}{A(x)}}$

Description of an Illustrative Embodiment

In an illustrative embodiment for a GMR-based system themagneto-resistive material consists of a Co/Cu multi-layer prepared bysputtering on a glass or silicon substrate with a thickness of thelayers of the order of 1 nm and chosen such that an anti-ferromagneticordering is obtained between consecutive magnetic layers. The number ofrepetitions is some tens and the multi-layer structure is protected by a1 nm thick coating layer of tantalum. This material is formed in theappropriate shape to achieve the desired sensitivity function either bymasking during deposition or by cutting or etching after deposition. Thesensitivity function is the result of a variation of one or twodimensions as displayed for a different geometry in FIGS. 2-5.

The obtained magneto-resistive material (the sensing element) is mountedfixed onto a holder and a small moving permanent magnet, rigidlyconnected to the moving object, the position of which is to bedetermined, is positioned close to the sensing element so that part ofthe sensor material is exposed to the magnetic field. The magnitude ofthe field from the permanent magnet is sufficiently large so that theexposed part of the sensing element is driven into the ferromagneticstate resulting in a (local) resistance change of the order of 20-50%.This change in resistance is measured directly or through the resultingasymmetry in a Wheatstone type bridge.

FIG. 1 illustrates a typical circuit diagram forming a bridge formeasuring the change in resistance of a position sensor element 1utilizing a magneto-resistive material. The sensor element 1 of aresistance Rx and a resistor 4 having a fixed value R form a firstbranch and a resistor 2 having a fixed value R1 and a resistor 3 havingthe fixed value R constitute the second branch of the bridge. Theresistance R1 corresponds to the nominal resistance of the sensorelement 1 and preferably having a temperature dependency correspondingto the temperature dependency of the sensor element 1. In a typicalembodiment a permanent magnet 5 is placed close to the sensor element 1such that the magnet and the element 1 may be displaced in relation toeach other in a x-direction indicated by the double arrow. One terminalof a voltmeter 6 is connected to the connection point between Rx and R1.The other terminal of the voltmeter 6 is connected to the connectionpoint between resistors 3 and 4. The voltmeter measures voltagedifferences achieved by the two voltage dividers formed by the twobranches, which are supplied by a voltage source E. Thus, a change inthe voltage difference displayed by the voltmeter 6 will be a functionof a variation of the resistance 1, which in turn is a function of amotion x of the magnet 5.

 R _(x) =R ₁ −∂R; ∂R=f(x)→V=V(x)

The area where the magnetic field acts is indicated by the referencenumeral 5 in FIG. 1. The element R1 and Rx may even be made as identicalelements. However in most cases R1 will be replaced by a suitablestandard metal film resistor. Furthermore the shape of the sensormaterial is varied to accommodate the specific sensitivity functiondesired.

Consequently the magneto-restive material is formed into an arbitraryshape to achieve the desired sensitivity function. The sensitivityfunction may primarily be the result of a variation in one dimension,e.g. the width of a strip of material as visualized by the form of Rx inFIG. 1. This is then accomplished by using a any type ofmagneto-resistive material where the constant thickness represents amulti layer structure having the thickness of the layer chosen such thatan anti-ferromagnetic ordering is obtained between consecutive magneticlayers.

According to the present improvement one dimension (width) or twodimensions (width and thickness) are varied, as is displayed in FIGS. 2and 3, respectively, where the number of repetitions of the layers isvaried while preserving the anti-ferromagnetic ordering, so that astepwise change is superimposed on the signal corresponding to thesimple wedge demonstrated in FIG. 2.

In FIGS. 4 and 5 further embodiments of the position sensitive sensorare displayed. A small moving magnet, rigidly connected to the movingobject the position of which is to be determined, is positioned close tothe magneto-resistive material so that part of the magneto-resistivematerial is exposed to the field and driven into the ferromagnetic stateresulting in a resistance change.

It will be understood by those skilled in the art that variousmodifications and changes may be made to the present invention withoutdeparture from the scope thereof, which is defined by the appendedclaims.

What is claimed is:
 1. A position sensor for non-contact positionmeasurement, the sensor comprising: a sensor body made of a giantmagneto-resistance (GMR) material with plural layers, said sensor bodyhaving a cross-sectional area and a number of said plural layers thatvary as a function of lengthwise distance along said sensor body toprovide a desired sensitivity function; and a sensor magnet adjacent tosaid sensor body and whose position relative to the lengthwise distancealong said sensor body defines an electrical resistance of said sensorbody, said sensor magnet being relatively movable lengthwise along saidsensor body to change the resistance.
 2. The position sensor of claim 1,wherein the cross-sectional area and the number of said plural layersvary due to a change in width of said sensor body as a function oflength.
 3. The position sensor of claim 1, wherein the cross-sectionalarea and the number of said plural layers vary due to a change in bothwidth and thickness of said sensor body as a function of length.
 4. Theposition sensor of claim 1, further comprising, first and second fixedresistance resistors connected in series, a third resistor connected inseries with said sensor body, said third resistor having a resistancethat corresponds to a nominal resistance of said sensor body, the seriesconnection of said first and second resistors being connected inparallel to the series connection of said third resistor and said sensorbody and to a source of electrical power, and a voltmeter connectedbetween a first node between said first and second resistors and asecond node between said third resistor and said sensor body, wherein avoltage measured by said voltmeter indicates a position of said sensormagnet.